A conventional shadow-mask-type CRT comprises an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multiapertured sheet of metal precisely disposed between the screen and the beam-producing means. The apertured metal sheet shadows the screen, and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color. A matrix of light-absorptive material surrounds the phosphor elements.
In one prior art process for forming each array of phosphor elements on a viewing faceplate of the CRT, the inner surface of the faceplate is coated with a slurry of a photosensitive binder and phosphor particles adapted to emit light of one of the three emission colors. The slurry is dried to form a coating and a light field is projected from a source through the apertures in the shadow mask and onto the dried coating so that the shadow mask functions as a photographic master. The exposed coating is subsequently developed to produce the first color-emitting phosphor elements. The process is repeated for the second and third color-emitting phosphor elements, utilizing the same shadow mask but repositioning the light source for each exposure. Each position of the light source approximates the convergence angle of one of the electron beams which excites the respective color-emitting phosphor elements. A more complete description of this prior art process, known as the photolithographic wet process, can be found in U.S. Pat. No. 2,625,734 issued to H. B. Law on Jan. 20, 1953.
A drawback of the above-described wet process is that the process may not be capable of meeting the higher resolution demands of the next generation of entertainment devices and the even higher resolution requirements for monitors, work stations and applications requiring color alpha-numeric text. Additionally, the wet photolithographic process (including matrix processing) requires 182 major processing steps (shown in FIGS. 1 and 2, with the number under each block indicating the number of stations required), necessitates extensive plumbing and the use of clean water, requires phosphor salvage and reclamation, and utilizes large quantities of electrical energy for exposing and drying the phosphor materials.
U.S. Pat. No. 3,475,169 issued to H. G. Lange on Oct. 28, 1969 discloses a process for electrophotographically screening color cathode-ray tubes. The inner surface of the faceplate of the CRT is coated with a volatilizable conductive material and then overcoated with a layer of volatilizable photoconductive material. The photoconductive layer is then uniformly charged, selectively exposed with light through the shadow mask to establish a latent charge image, and developed using a high molecular weight carrier liquid bearing, in suspension, a quantity of phosphor particles of a given emissive color that are selectively deposited onto suitably charged areas of the photoconductive layer to develop the latent image. The charging, exposing and deposition process is repeated for each of the three color-emissive phosphors, i.e., green, blue, and red, of the screen. An improvement in electrophotographic screening is described in U.S. Pat. No. 4,448,866 issued to H. G. Olieslagers, et al. on May 15, 1984. In the latter patented process, phospher particle adhesion is said to be increased by uniformly exposing, with light, the portions of the photoconductive layer lying between the deposited pattern of phosphor particles after each deposition step to reduce or discharge any residual charge and to permit a more uniform recharging of the photoconductor for subsequent depositions. Since the latter two patents disclose an electrophotographic process that is, in essence, a wet process, many of the drawbacks described above, with respect to the wet photolithographic process of U.S. Pat. No. 2,625,734 also are applicable to the wet electrophotographic process.
The process of the present invention is a dry electrophotographic process which eliminates or minimizes many of the drawbacks of the prior art processes.